The subject matter disclosed herein relates generally to isotope production systems, and more particularly to isotope production systems having liquid targets that are irradiated with a particle beam.
Radioisotopes (also called radionuclides) have several applications in medical therapy, imaging, and research, as well as other applications that are not medically related. Systems that produce radioisotopes typically include a particle accelerator, such as a cyclotron, that accelerates a beam of charged particles (e.g., H− ions) and directs the beam into a target material to generate the isotopes. The cyclotron includes a particle source that provides the particles to a central region of an acceleration chamber. The cyclotron uses electrical and magnetic fields to accelerate and guide the particles along a predetermined orbit within the acceleration chamber. The magnetic fields are provided by electromagnets and a magnet yoke that surrounds the acceleration chamber. The electrical fields are generated by a pair of radio frequency (RF) electrodes (or lees) that are located within the acceleration chamber. The RF electrodes are electrically coupled to an RF power generator that energizes the RF electrodes to provide the electrical field. The electrical and magnetic fields cause the particles to take a spiral-like orbit that has an increasing radius. When the particles reach an outer portion of the orbit, the particles may form a particle beam that is directed toward the target material for isotope production.
Target material (also referred to as starting material) is typically housed within a chamber of a target assembly that is positioned within the path of the particle beam. In some systems, the target material is a liquid (hereinafter referred to as a target liquid). The chamber may be defined by a recess within a target body and a foil that covers the recess. The particle beam is incident on the foil and the target liquid within the chamber. The particle beam deposits a relatively large amount of power (e.g., 1-2 kW) within a relatively small volume of the target liquid (e.g., 1-3 ml). The thermal energy generated within the chamber drives the target liquid to a boiling state. Consequently, bubbles are generated within the target liquid along a surface of the foil or from within the volume of the target liquid.
The bubbles may cause some unwanted effects. For example, the production chamber is typically divided into a liquid region and a gas or vapor region, which is positioned above the liquid region. The bubbles generated within the liquid region eventually rise to the gas region. When a greater proportion of bubbles exists within the liquid region, the bubbles may permit the particle beam to travel completely through the liquid region without causing the desired changes to the isotopes of the target liquid. As such, the bubbles may reduce the efficiency of radioisotope production. Furthermore, a greater proportion of bubbles within the liquid region may reduce the target liquid's ability to absorb thermal energy from the foil. It may be necessary to more frequently replace or refurbish the target assembly.
Conventional methods of reducing bubble formation include cooling the production chamber by flowing liquid or gas through channels that are proximate to the production chamber. Bubble formation may also be reduced by pressurizing the production chamber with an inert gas, such as helium or argon. Such methods, however, may have only a limited effectiveness.